The field of the invention is magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to quantifying and classifying MRI images of atherosclerotic plaques (coronary artery, carotid artery, aorta, etc.).
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Coronary atherosclerosis is the main contributing factor to ischemic heart disease, the leading cause of mortality in developed countries. Characterization of atherosclerotic plaque structure in coronary arteries is an important factor in assessing susceptibility to acute coronary syndromes. Noninvasive computed tomography (CT) may provide differentiation between calcified and non-calcified plaques in patients at the expense of ionizing radiation exposure. Noninvasive high-resolution MRI has been used in-vivo to monitor changes in burden and composition of human carotid arteries and aortic plaques. Multicontrast MRI as disclosed in U.S. Pat. No. 4,945,478 has been used for classification of atherosclerotic plaque components. Qualitative assessment of plaque composition using pattern recognition was performed on human aortas using T1, T2, and proton density weighted MR images. Normal wall (media), lipid-rich plaques, and fibrous plaques were differentiated with a clustering algorithm.
Multicontrast MRI methods employ different pulse sequences to obtain multiple images of the subject artery. Each pulse sequence is chosen to “weight” the reconstructed magnitude image in such a manner that it highlights, or images, one of the atherosclerotic tissue class types involved in the disease process. To evaluate a disease such as atherosclerosis requires combining together structural information as well as functional information. Structural information includes characteristics of the vessel such as its shape in three dimensions, its location with respect to other structures, and its degree of vessel occlusion, or narrowing. Such structural information may be obtained from any number of different MRI pulse sequences. Functional information involves identification and quantification of the various soft tissue types involved in the disease process. Functional information derived from multicontrast MR images provides information regarding the physiological stage of the disease process.